1. Field of the Invention
The present invention relates to an overvoltage protective module for establishing a grounding path from a circuit, such as a line, to a grounding contact and also for testing one section of the circuit with respect to ground.
2. Description of the Prior Art
An overvoltage protective module contains an overvoltage protective or a so-called "open gap" protector to protect one wire of a line against undesirable high overvoltage, such as that which results from lightning. This wire may, for example, be formed by a pair of conductors, one of which is connected to the inner side of an exchange frame for incoming subscriber's telephone lines and an other of which is connected to a terminal of a telephone exchange. The two conductors are connected to ground via the overvoltage protector in normal operation. Thus, by definition, a protector of this type for protection against overvoltage is connected in parallel between a line wire and a grounding terminal, in contrast to overcurrent protective elements, such as fuses, which have to be connected in series on the wire line and which are not included in the scope of the present invention.
U.S. Pat. No. 2,546,824 discloses a moulded plastic block which contains a cylindrical spaced gap protector with two carbon electrodes. The terminals of the line wire are formed as helical springs which are set coaxially in the cavity of the block and exert pressure on elements of a protector assembly, thereby to normally maintain the elements of the assembly in a compactly assembled relationship. The end of a first spring which is connected to the terminal on the side of the telephone line is applied against the second terminal of the spaced gap protector by means of a metallic member. The end of a second spring which is connected to the terminal at the telephone exchange is applied against a metallic end cap which is fixed to an insulating material housing containing a coil. This cap is connected to the metallic member under the effect of the second spring via a conducting wire, a winding and a metallic sleeve which are anchored to the cap, and via a low melting-point solder between the sleeve and a coaxial pin in the block. The first terminal of the spaced gap protector is inserted in a bayonet cap whose lugs slide into the grooves of a drilled metal face plate connected to ground.
The bayonet closing means are conventional. The bayonet cap initially translated horizontally along the longitudinal axis of the spaced gap protector then it rotates in order to anchor and hold all the members in the block. The result of this is that there is no stationary position in which the spaced gap protector is only connected to the terminal of the outer circuit on the side of the telephone line and disconnected from the terminal of the inner circuit on the side of the telephone exchange, owing to the force exerted by the helical springs in the direction of the bayonet cap. Thus, the individual ground insulation of the outer circuit cannot be tested. In addition, the helical springs are required to extract the members included in the block.
U.S. Pat. No. 3,825,867 discloses an electrical protective module in which an overvoltage protective element or spark-gap device can be inserted sideways in a block of the module and comprises a terminal which is connected to a grounding terminal strap by means of a coil spring, and another terminal which is connected to an elongated terminal strap connected to a wire of a subscriber's telephone line by means of a heating coil and a side-mounted contact plate which is rigid with this coil. Interruptive means in the form of a movable detent actuator made of insulating material may slide between the contact plate and the contact end surface of an elongated strap forming another terminal of the line. Thus, in the absence of the detent actuator, the line is closed regardless of whether the gap-spack device is present or not. When the detent actuator is inserted, it interrupts the electrical conductive path between the terminals of the line wire. In addition, the terminal strap associated with the subscriber's line is permanently connected to the spark-gap device.